Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.). Group IV semiconductors generally refer to those elements in the fourth column of the periodic table (e.g., carbon, silicon, germanium, etc.).
In general, a solid semiconductor tends to exist in three forms: crystalline, polycrystalline, and amorphous. In crystalline form, semiconductor atoms are positioned in a single unbroken crystal lattice with no grain boundaries. In polycrystalline form, the semiconductor atoms are positioned in many smaller and randomly oriented crystallites (smaller crystals). The crystallites are often referred to as grains. In amorphous form, the semiconductor atoms show no long-range positional order.
In general, conduction generally refers to the movement of electrically charged carriers, such as electrons or holes (i.e., lack of electrons), through electromagnetic fields. Metals tend to have substantial amounts of electrically charged particles available, whereas insulators have very few.
In the absence of impurities (called dopants), a semiconductor tends to behave as insulator, inhibiting the flow of an electric current. However, after the addition of relatively small amounts of dopants, the electrical characteristics of a semiconductor can dramatically change to a conductor by increasing the amount of electrically charged carriers. For example, in a process called photoexcitation, absorbed light will generally create an electron-hole pair (photocarriers) that in turn tends to increase overall conductivity (photoconductivity).
Depending on the kind of impurity, a doped region of a semiconductor can have more electrons (n-type) or more holes (p-type). For example, in a common configuration, a p-type region is placed next to an n-type region in order to create a (p-n) junction with a “built-in” potential. That is, the energy difference between the two Fermi levels.
Under generally accepted principles of quantum mechanics, electrons of an atom can only reside in certain states, so that only particular energy levels are possible. However, the occupation of particular energy states cannot be determined with particularity. Consequently, for an ensemble of atoms (e.g., solid) a probability distribution or density is commonly used, called the Fermi level. In general, the Fermi level describes the energy level at given temperature in which ½ of the energy states are filled. Energy states are unique and correspond to a quantum number.
Consequently, electrons on the p-type side of the junction within the electric field may then be attracted to the n-type region and repelled from the p-type region, whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
There are several methods of doping a semiconductor. One method involves depositing a doped glass on a semiconductor substrate, such as a Si wafer. Once exposed to relatively high temperature (e.g., 800-1100° C.), the dopants will tend to diffuse from the highly-doped glass into the substrate.
In addition, the high temperature also tends to anneal the substrate. Annealing is generally the process of heating a material above a certain critical temperature in order to reduce the materials internal stresses, and or improve its physical and electrical properties. In the case of a semiconductor substrate, annealing allows the dopant atoms to properly diffuse (from a high to a lower concentration region) and position themselves in the lattice, such that the additional electrons or holes (which arise from the n-type and p-type dopants, respectively) are available for the transmission of current. This is generally called activation (or effectiveness of “donation”) and is critical for the creation of an efficient p-n junction.
There are several methods of doping a semiconductor. However, most of these may be problematic. For example, a common method involves depositing a doped glass on a semiconductor substrate via a silk-screen. A printing technique that makes use of a squeegee, silk-screening mechanically forces a liquid, such as a highly doped glass paste, directly onto a substrate. After drying the liquid the wafer is placed in a conveyor belt passing through the furnace. The temperature inside it can be adjusted in several zones and, though the furnace is open, gases can be supplied. A cycle begins with several minutes at around 600° C. with clean air to burn off the organic materials of the paste, followed by the diffusion step at relatively high temperatures (e.g., 800-1100° C.), where the dopants tend to diffuse from the highly-doped glass into the substrate. The high temperature will also tend to anneal the substrate.
However, the downward mechanical force of the squeegee also tends to subject the substrate to stress, and hence may detrimentally affect the electrical and physical characteristics of the substrate. For devices that required multiple deposition steps, such as a back contact solar cell, the stress is aggravated. In general, every additional screen printing step tends to reduce the process yield (and increase costs) due to damage or breakage. Additionally, alignment of the screen pattern may also present substantial challenges. For example, if pattern alignment is poor, the resulting solar cells may malfunction (short) further reducing process yield.
In an alternate doping method, dopants may be deposited in a crystalline or polycrystalline substrate through ion implantation. Ion implantation generally accelerates dopant ions into the substrate at high energy. Like diffusion doping, the substrate must also generally be annealed at a high temperature to repair the substrate and activate the dopants. However, although dopant dosage may be controlled with high precision, ion implantation tends to be very expensive since it requires the use of specialized and expensive semiconductor manufacturing equipment.
Likewise, the use of chemical vapor deposition (CVD) to add dopants may also have drawbacks. In a typical CVD process, a substrate (which can be an insulator, a semiconductor, or metal) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce a doped film. However, like ion implantation, CVD is expensive since it requires specialized and expensive semiconductor manufacturing equipment. In addition, CVD also tends to be very slow, as the film layers are built up a single atom at a time.
Other common doping techniques include gas phase doping. In these the cells to be diffused, loaded in quartz boats, are placed in a quartz tube with resistance heating and held at the processing temperature. The cells enter and exit the furnace through one end, while gases are fed through the opposite one. Dopant itself can be supplied in this way, typically by bubbling nitrogen through liquid dopant precursor before injection into the furnace. Solid dopant sources are also compatible with furnace processing. Five to fifteen minutes at temperatures in the range from about 900° C. to about 950° C. can be considered representative. These methods however suffer from lack of ability to pattern simultaneous p-type and n-type doping. In addition this process does not comply with requirements of in-line processing and may have limited manufacturing throughput.
In view of the foregoing, there is desired a method of producing Group IV nanoparticle junctions and devices therefrom.